Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor, and an oxide semiconductor has been attracting attention as well.
For example, Patent Document 1 discloses a technique to reduce the cell area by stacking a plurality of memory cells comprising a first transistor including an oxide semiconductor film and a second transistor including an oxide semiconductor film.
Patent Document 2 discloses a technique to reduce the area of a driver circuit portion in the vicinity of a pixel portion by stacking a first layer including the driver circuit portion for operating the plurality of pixels and a second layer including the pixel portion including a plurality of two-dimensionally arranged pixels.